Reduction cells for reducing alumina to aluminum require adequate insulation in the cathode to limit heat losses from the steel shell surrounding the cathode during cell operation. Cryolitic salts and vapors, containing an excess of sodium fluoride, penetrate through the carbonaceous cathode lining and chemically attack and degrade the thermal insulation in the bottom of the cathode shell. As the insulation is degraded, the insulation loses its effectiveness as a thermal insulation material and heat losses through the insulation increase. As a consequence, the cell voltage must often be increased in order to maintain a stable thermal equilibrium in the cell. If this is not done, the temperature of the electrolyte decreases, resulting in increases in anode effects and/or reduced interelectrode distance in the cell. Either of these consequences results in reduced productivity and/or increased operating costs for the cell.
The highly corrosive cryolitic salt vapors penetrating through the cathode lining can be stopped in either of two ways. If the temperature isotherm through the insulation is maintained sufficiently low, i.e., less than about 600.degree. C., to prevent any mobility and thereby any reactivity of the salts above their freezing points, these salts do not migrate to the insulation layers. This is, however, an extremely inefficient way to operate an alumina reduction cell, and thus the benefits to be gained by prohibiting cryolitic salt vapor penetration to the insulation are more than offset by other system inefficiencies.
The other alternative method to protect alumina reduction cell insulation from cryolitic salt vapors is by means of a physical barrier material. This barrier is positioned above the low temperature insulation material and beneath higher temperature insulation material which is in contact with the carbon cathode and cell bottom wall. Numerous materials have been tried in the past as a barrier layer, with the most common material being a steel plate. It is difficult, however, to obtain a unitary steel plate of a size sufficient to cover an alumina reduction cell and gaps between steel plates which are laid next to one another to form a sufficient size barrier layer provide regions for vapor penetration.
In U.S. Pat. No. 4,411,758 a low softening point glass, such as soda-lime glass, typically in the form of cullet, is disclosed as a physical barrier layer for alumina reduction cells. It has been found, however, that such a soda-lime glass layer is ineffective as a barrier material in resisting cryolitic salts and vapors in reduction cell cathodes. Its chemical reactivity, due to its chemical composition, and its relatively low softening point, flow point and surface tension, make this material difficult to contain in the alumina reduction cell at normal operating temperatures, i.e., at or above about 900.degree. C., during cell operation as well as providing less than desired protection for the insulation material.
There remains a need, therefore, to provide a reliable alumina reduction cell barrier material.